It is common practice in the petroleum industry to employ so-called packing devices in the bore of an oil or gas well to isolate one or more portions of the well bore for purposes of testing, treating or producing the well. Common treating operations to enhance production from a well include, but are not limited to, acidizing and fracturing. Steam injection, water injection or injection of a gas such as carbon dioxide (CO.sub.2) may also be termed as "treating" operations used to enhance the production of depleted wells or those producing heavy crude. In testing, treating and producing operations, pack-off devices, which are commonly known as "packers" or "bridge plugs," may be subjected to extremes of high temperature and pressure, in combination with corrosive fluids such as the variety of acids employed in acidizing and fracturing operations, water, steam or CO.sub.2 injection fluids, and hydrogen sulfide (H.sub.2 S), brine and other well fluids.
These extremely hostile downhole environments must be accommodated with packers and bridge plugs designed to provide effective, leak-free seals over long periods of time, which may extend into years. In particular, when such devices are employed in producing a well, or in injecting fluids into a well on a long-term basis such as in waterflood, steam or CO.sub.2 injection projects, it is expensive both in terms of rig costs and in lost production volume to have to replace them after a relatively short period of time. The problem is, of course, compounded in large fields where hundreds or even thousands of wells are being produced or fluid injected therein. Therefore, it is desirable to employ packers which are rugged, corrosion-resistant, relatively inexpensive, simple to set as well as to retrieve (when necessary), and which create a long-lasting and leakfree seal across the well bore.
Examples of a variety of retrievable prior art pack-off devices are disclosed in U.S. Pat. Nos. 3,244,233, 3,507,327, 3,584,684, 3,749,166 and 4,078,606. As may readily be seen, however, these prior art pack-off devices are relatively complex in design and construction and leave a large number of parts exposed to the hostile well environment below the device.